Inverted organic photosensitive devices

ABSTRACT

The present disclosure relates to organic photosensitive optoelectronic devices grown in an inverted manner. An inverted organic photosensitive optoelectronic device of the present disclosure comprises a reflective electrode, an organic donor-acceptor heterojunction over the reflective electrode, and a transparent electrode on top of the donor-acceptor heterojunction.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims priority to U.S. ProvisionalPatent Application No. 61/108,817, entitled “Inverted OrganicPhotovoltaics,” filed Oct. 27, 2008, and U.S. Provisional PatentApplication No. 61/109,305, entitled “Inverted Organic Photovoltaics,”filed Oct. 29, 2008, the entire contents of both of which areincorporated herein by reference in their entireties for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-FG36-08G018022awarded by the Department of Energy and FA9550-07-1-0364 awarded by theAir Force Office of Scientific Research. The government has certainrights in the invention.

JOINT RESEARCH AGREEMENT

The subject matter of the present disclosure was made by, on behalf of,and/or in connection with one or more of the following parties to ajoint university-corporation research agreement: Princeton University,The University of Michigan, and Global Photonic Energy Corporation. Theagreement was in effect on and before the date the subject matter of thepresent disclosure was made, and such was made as a result of activitiesundertaken within the scope of the agreement.

FIELD

The present disclosure generally relates to organic photosensitiveoptoelectronic devices. More specifically, it is directed to organicphotosensitive optoelectronic devices grown in an inverted manner,comprising a reflective substrate and transparent top electrode.

BACKGROUND

Optoelectronic devices rely on the optical and electronic properties ofmaterials to either produce or detect electromagnetic radiationelectronically or to generate electricity from ambient electromagneticradiation. Photosensitive optoelectronic devices convert electromagneticradiation into electricity. Solar cells, also called photovoltaic (PV)devices, are a type of photosensitive optoelectronic device that isspecifically used to generate electrical power. PV devices, which maygenerate electrical energy from light sources other than sunlight, canbe used to drive power consuming loads to provide, for example, lightingor heating, or to power electronic circuitry or devices such ascalculators, radios, computers or remote monitoring or communicationsequipment. These power generation applications also often involve thecharging of batteries or other energy storage devices so that operationmay continue when direct illumination from the sun or other lightsources is not available, or to balance the power output of the PVdevice with a specific application's requirements. As used herein theterm “resistive load” refers to any power consuming or storing circuit,device, equipment or system.

Another type of photosensitive optoelectronic device is a photoconductorcell. In this function, signal detection circuitry monitors theresistance of the device to detect changes due to the absorption oflight.

Another type of photosensitive optoelectronic device is a photodetector.In operation, a photodetector is used in conjunction with a currentdetecting circuit which measures the current generated when thephotodetector is exposed to electromagnetic radiation and may have anapplied bias voltage. A detecting circuit as described herein is capableof providing a bias voltage to a photodetector and measuring theelectronic response of the photodetector to electromagnetic radiation.

These three classes of photosensitive optoelectronic devices may becharacterized according to whether a rectifying junction, as definedbelow, is present and also according to whether the device is operatedwith an external applied voltage, also known as a bias or bias voltage.A photoconductor cell does not have a rectifying junction and isnormally operated with a bias. A PV device has at least one rectifyingjunction and is operated with no bias. A photodetector has at least onerectifying junction and is usually but not always operated with a bias.Typically, a PV cell may provide power to a circuit, device orequipment, but may not provide a signal or current to control detectioncircuitry, or the output of information from the detection circuitry. Incontrast, a photodetector or photoconductor provides a signal or currentto control detection circuitry, or the output of information from thedetection circuitry but does not provide power to the circuitry, deviceor equipment.

Traditionally, photosensitive optoelectronic devices have beenconstructed of a number of inorganic semiconductors, e.g., crystalline,polycrystalline and amorphous silicon, gallium arsenide, cadmiumtelluride and others. Herein, the term “semiconductor” denotes materialsthat can conduct electricity when charge carriers are induced by thermalor electromagnetic excitation. The term “photoconductive” generallyrelates to the process in which electromagnetic radiant energy isabsorbed and thereby converted to excitation energy of electric chargecarriers so that the carriers can conduct, i.e., transport, electriccharge in a material. The terms “photoconductor” and “photoconductivematerial” are used herein to refer to semiconductor materials which arechosen for their property of absorbing electromagnetic radiation togenerate electric charge carriers.

PV devices may be characterized by the efficiency with which they canconvert incident solar power to useful electric power. Devices utilizingcrystalline or amorphous silicon dominate commercial applications, andsome have achieved efficiencies of 23% or greater. However, efficientcrystalline-based devices, especially of large surface area, aredifficult and expensive to produce due to the problems inherent inproducing large crystals without significant efficiency-degradingdefects. On the other hand, high efficiency amorphous silicon devicesstill suffer from problems with stability. Present commerciallyavailable amorphous silicon cells have stabilized efficiencies between 4and 8%. More recent efforts have focused on the use of organic PV cellsto achieve acceptable photovoltaic conversion efficiencies witheconomical production costs.

PV devices may be optimized for maximum electrical power generationunder standard illumination conditions (i.e., Standard Test Conditionswhich are 1000 W/m², AM1.5 spectral illumination), for the maximumproduct of photocurrent times photovoltage. The power conversionefficiency of such a cell under standard illumination conditions dependson the following three parameters: (1) the current under zero bias,i.e., the short-circuit current (J_(SC), in Amperes (2) the photovoltageunder open circuit conditions, i.e., the open circuit voltage (V_(OC)),in Volts (V) and (3) the fill factor, FF.

PV devices produce a photo-generated current when they are connectedacross a load and are irradiated by light. When irradiated underinfinite load, a PV device generates its maximum possible voltage, orV_(OC). When irradiated with its electrical contacts shorted, a PVdevice generates its maximum possible current, I short-circuit, orI_(SC). When actually used to generate power, a PV device is connectedto a finite resistive load and the power output is given by the productof the current and voltage, I×V. The maximum total power generated by aPV device is inherently incapable of exceeding the product,I_(SC)×V_(OC). When the load value is optimized for maximum powerextraction, the current and voltage have the values, I_(max) andV_(max), respectively.

A figure of merit for PV devices is the fill factor, FF, defined as:

FF={I _(max) V _(max) }/{I _(SC) V _(OC)}  (1)

where FF is always less than 1, as I_(SC) and V_(OC) are never obtainedsimultaneously in actual use. Nonetheless, as FF approaches 1, thedevice has less series or internal resistance and thus delivers agreater percentage of the product of I_(SC) and V_(OC) to the load underoptimal conditions. Where P_(inc) is the power incident on a device, thepower efficiency of the device, η_(P), may be calculated by:

η_(P)=FF*(I _(SC) *V _(OC))/P _(inc)

When electromagnetic radiation of an appropriate energy is incident upona semiconductive organic material, for example, an organic molecularcrystal (OMC) material, or a polymer, a photon can be absorbed toproduce an excited molecular state. This is represented symbolically asS_(θ)+hVΨS₀*. Here S₀ and S₀* denote ground and excited molecularstates, respectively. This energy absorption is associated with thepromotion of an electron from a bound state in the Highest OccupiedMolecular Orbital (HOMO) energy level, which may be a B-bond, to theLowest Unoccupied Molecular Orbital (LUMO) energy level, which may be aB-bond, or equivalently, the promotion of a hole from the LUMO energylevel to the HOMO energy level. In organic thin-film photoconductors,the generated molecular state is generally believed to be an exciton,i.e., an electron-hole pair in a bound state which is transported as aquasi-particle. The excitons can have an appreciable life-time beforegeminate recombination, which refers to the process of the originalelectron and hole recombining with each other, as opposed torecombination with holes or electrons from other pairs. To produce aphotocurrent, the electron-hole pair becomes separated, typically at adonor-acceptor interface between two dissimilar contacting organic thinfilms. If the charges do not separate, they can recombine in a geminantrecombination process, also known as quenching, either radiatively, bythe emission of light of a lower energy than the incident light, ornon-radiatively, by the production of heat. Either of these outcomes isundesirable in a photosensitive optoelectronic device.

Electric fields or inhomogeneities at a contact may cause an exciton toquench rather than dissociate at the donor-acceptor interface, resultingin no net contribution to the current. Therefore, it may be desirable tokeep photogenerated excitons away from the contacts. This has the effectof limiting the diffusion of excitons to the region near the junction sothat the associated electric field has an increased opportunity toseparate charge carriers liberated by the dissociation of the excitonsnear the junction.

To produce internally generated electric fields which occupy asubstantial volume, the usual method is to juxtapose two layers ofmaterial with appropriately selected conductive properties, especiallywith respect to their distribution of molecular quantum energy states.The interface of these two materials is called a PV heterojunction. Intraditional semiconductor theory, materials for forming PVheterojunctions have been denoted as generally being of either n or ptype. Here, n-type denotes that the majority carrier type is theelectron. This could be viewed as the material having many electrons inrelatively free energy states. The p-type denotes that the majoritycarrier type is the hole. Such material has many holes in relativelyfree energy states. The type of the background, i.e., notphoto-generated, majority carrier concentration depends primarily onunintentional doping by defects or impurities. The type andconcentration of impurities determine the value of the Fermi energy, orlevel, within the gap between the HOMO energy level and LUMO energylevel, called the HOMO-LUMO gap. The Fermi energy characterizes thestatistical occupation of molecular quantum energy states denoted by thevalue of energy for which the probability of occupation is equal to ½. AFermi energy near the LUMO energy level indicates that electrons are thepredominant carrier. A Fermi energy near the HOMO energy level indicatesthat holes are the predominant carrier. Accordingly, the Fermi energy isa primary characterizing property of traditional semiconductors and theprototypical PV heterojunction has traditionally been the p-n interface.

The term “rectifying” denotes, inter alia, that an interface has anasymmetric conduction characteristic, i.e., the interface supportselectronic charge transport preferably in one direction. Rectificationis associated normally with a built-in electric field which occurs atthe heterojunction between appropriately selected materials.

As used herein, and as would be generally understood by one skilled inthe art, a first HOMO or LUMO energy level is “greater than” or “higherthan” a second HOMO or LUMO energy level if the first energy level iscloser to the vacuum energy level. Since ionization potentials (IP) aremeasured as a negative energy relative to a vacuum level, a higher HOMOenergy level corresponds to an IP having a smaller absolute value (an IPthat is less negative). Similarly, a higher LUMO energy levelcorresponds to an electron affinity (EA) having a smaller absolute value(an EA that is less negative). On a conventional energy level diagram,with the vacuum level at the top, the LUMO energy level of a material ishigher than the HOMO energy level of the same material. A “higher” HOMOor LUMO energy level appears closer to the top of such a diagram than a“lower” HOMO or LUMO energy level.

In the context of organic materials, the terms “donor” and “acceptor”refer to the relative positions of the HOMO and LUMO energy levels oftwo contacting but different organic materials. This is in contrast tothe use of these terms in the inorganic context, where “donor” and“acceptor” may refer to types of dopants that may be used to createinorganic n- and p-types layers, respectively. In the organic context,if the LUMO energy level of one material in contact with another islower, then that material is an acceptor. Otherwise it is a donor. It isenergetically favorable, in the absence of an external bias, forelectrons at a donor-acceptor junction to move into the acceptormaterial, and for holes to move into the donor material.

A significant property in organic semiconductors is carrier mobility.Mobility measures the ease with which a charge carrier can move througha conducting material in response to an electric field. In the contextof organic photosensitive devices, a layer including a material thatconducts preferentially by electrons due to a high electron mobility maybe referred to as an electron transport layer, or ETL. A layer includinga material that conducts preferentially by holes due to a high holemobility may be referred to as a hole transport layer, or HTL. In somecases, an acceptor material may be an ETL and a donor material may be anHTL.

Conventional inorganic semiconductor PV cells may employ a p-n junctionto establish an internal field. However, it is now recognized that inaddition to the establishment of a p-n type junction, the energy leveloffset of the heterojunction may also play an important role. The energylevel offset at the organic donor-acceptor (D-A) heterojunction isbelieved to be important to the operation of organic PV devices due tothe fundamental nature of the photogeneration process in organicmaterials. Upon optical excitation of an organic material, localizedFrenkel or charge-transfer excitons are generated. For electricaldetection or current generation to occur, the bound excitons must bedissociated into their constituent electrons and holes. Such a processcan be induced by the built-in electric field, but the efficiency at theelectric fields typically found in organic devices (F˜10⁶ V/cm) is low.The most efficient exciton dissociation in organic materials occurs at aD-A interface. At such an interface, the donor material with a lowionization potential forms a heterojunction with an acceptor materialwith a high electron affinity. Depending on the alignment of the energylevels of the donor and acceptor materials, the dissociation of theexciton can become energetically favorable at such an interface, leadingto a free electron polaron in the acceptor material and a free holepolaron in the donor material.

Organic PV cells have many potential advantages when compared totraditional silicon-based devices. Organic PV cells are light weight,economical in materials use, and can be deposited on low costsubstrates, such as flexible plastic foils. However, organic PV devicestypically have relatively low quantum yield (the ratio of photonsabsorbed to carrier pairs generated, or electromagnetic radiation toelectricity conversion efficiency), being on the order of 1% or less.This is, in part, thought to be due to the second order nature of theintrinsic photoconductive process. That is, carrier generation requiresexciton generation, diffusion and ionization or collection. There is anefficiency η associated with each of these processes. Subscripts may beused as follows: P for power efficiency, EXT for external quantumefficiency, A for photon absorption, ED for diffusion, CC forcollection, and INT for internal quantum efficiency. Using thisnotation:

η_(P)˜η_(EXT)=η_(A)*η_(ED)*η_(CC)

η_(EXT)=η_(A)*η_(INT)

The diffusion length (L_(D)) of an exciton is typically much less(L_(D)˜50 Å) than the optical absorption length (˜500 Å), requiring atrade off between using a thick, and therefore resistive, cell withmultiple or highly folded interfaces, or a thin cell with a low opticalabsorption efficiency.

Conventional organic PV cells are fabricated on transparent substratessuch as glass or plastic coated with a transparent conductor, such asindium tin oxide (ITO). Because these substrates can be expensive and/oran important element of the overall cost structure of the device, theuse of such transparent conducting substrates has the potential to limitthe cost-effectiveness of the overall device, especially in large-areaapplications. Inverted organic PV cells utilize a reflective substrateand a transparent top contact. This architecture eliminates the need forcomparatively high-cost transparent substrates and allows forfabrication on arbitrary surfaces. This design significantly extends theapplication of organic PV cells, such as allowing for power-generatingcoatings or growth on flexible and inexpensive opaque substrates, suchas, for example, metal foil. Accordingly, there exists a need to developmore efficient inverted organic photosensitive structures.

SUMMARY

The present disclosure relates to organic photosensitive optoelectronicdevices, such as organic PV devices, grown in an inverted manner. Forpurposes of this disclosure, growth in an inverted manner means startingwith a reflective electrode and using a transparent top electrode. Insome embodiments, the inverted organic PV devices described hereincomprise:

a reflective electrode;

an organic donor-acceptor heterojunction over said reflective electrode;and

a transparent electrode over said donor-acceptor heterojunction.

In some embodiments, the reflective electrode may comprise an substrate,such as a metal anode. In some embodiments, the electrode may comprise alow work function metal selected from steel, Ni, Ag, Al, Mg, In, andmixtures or alloys thereof.

In certain embodiments, the inverted organic PV devices described hereincomprise: a surface-treated reflective electrode; an organicdonor-acceptor heterojunction over said reflective electrode; and atransparent electrode over said donor-acceptor heterojunction.

In some embodiments, the donor of the organic donor-acceptorheterojunction may be selected from phthalocyanines, porphyrins,subphthalocyanines, and derivatives or transition metal complexesthereof. In some embodiments, the donor comprises copper phthalocyanine(CuPc). In some embodiments, the acceptor of the organic donor-acceptorheterojunction is chosen from polymeric or non-polymeric perylenes,polymeric or non-polymeric naphthalenes, and polymeric or non-polymericfullerenes. In some embodiments, the acceptor comprises3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI).

In some embodiments, the transparent electrode is chosen fromtransparent oxides and metal or metal substitutes having a thicknesssufficient to render them transparent or semi-transparent. In someembodiments, the transparent electrode is selected from transparentconducting oxides such as indium tin oxide (ITO), gallium indium tinoxide (GITO), and zinc indium tin oxide (ZITO).

In some embodiments, the inverted organic PV devices described hereinmay optionally comprise one or more blocking layers, such as an excitonblocking layer (EBL), between the reflective electrode and thetransparent electrode. In some embodiments, the EBL may be selected fromN,N′-diphenyl-N,N′-bis-alpha-naphthylbenzidine (NPD), aluminum tris(8-hydroxyquinoline) (Alq3), carbazole biphenyl (CBP), bathocuproine(BCP), and tris(acetylacetonato) ruthenium (III) (Ru(acac)₃).

Also described herein are power-generating devices comprising at leastone organic PV device which comprises:

a reflective electrode;

an organic donor-acceptor heterojunction over said reflective electrode;and

a transparent electrode over said donor-acceptor heterojunction.

In some embodiments, the power-generating devices are formed on asubstrate film or a foil. In some embodiments, the power generatingdevice is formed directly on the enclosure of a device, wherein thedevice enclosure functions as a substrate and the reflective electrodeis formed over the substrate.

A method for producing an organic PV device is also described,comprising:

providing a reflective electrode;

performing at least one surface treatment on said reflective electrode;

forming an organic donor-acceptor heterojunction over said reflectiveelectrode; and

forming a transparent electrode over said organic donor-acceptorheterojunction.

Also described are methods for generating and/or measuring electricity.In some embodiments, the method comprises:

providing light to an organic PV device comprising

-   -   a reflective electrode;    -   an organic donor-acceptor heterojunction over said reflective        electrode; and    -   a transparent electrode over said donor-acceptor heterojunction.

In some embodiments the substrate is reflective, such as, for example, ametal foil, and the electrode closest to said reflective substrate isformed from suitable transparent or semitransparent materials definedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an inverted organic PV device comprising a reflectiveelectrode formed over a substrate, an organic donor-acceptorheterojunction on top of said reflective electrode, and a transparentelectrode on top of said donor-acceptor heterojunction.

FIG. 2a is a plot of PTCBI thicknesses versus eta (η_(P)—powerconversion efficiency) and responsivity (J_(SC)/P₀), wherein A/Wconnotes Amps per Watt and “sim” connotes “simulated.”

FIG. 2b is a plot of PTCBI versus V_(OC) and FF.

FIG. 2c is a plot of PTCBI thickness versus series resistance (R_(S))and n.

FIG. 2d is a plot of PTCBI thickness versus reverse saturation current(J_(S)).

FIG. 3a is a plot of CuPc thicknesses versus eta and J_(SC)/P₀.

FIG. 3b is a plot of CuPc versus V_(OC) and FF.

FIG. 3c is a plot of CuPc thickness versus R_(S) and.

FIG. 3d is a plot of CuPc thickness versus J_(S).

FIG. 4a shows calculations from a standard transfer-matrix simulationperformed on a control PV device grown on glass: ITO (1550 Å)/CuPc (200Å)/PTCBI (250 Å)/BCP (100 Å)/Ag (1000 Å). The optical fields at peakabsorption of CuPc at 625 nm (squares) and of PTCBI at 540 nm (stars)are shown.

FIG. 4b shows calculations from a standard transfer-matrix simulationperformed on an inverted PV device consistent with the embodimentsdescribed herein: quartz/Ag (1000 Å)/BCP (100 Å)/PTCBI (300 Å)/CuPc (150Å)/ITO (400 Å). The optical fields at peak absorption of CuPc at 625 nm(squares) and of PTCBI at 540 nm (stars) are shown.

FIG. 4c shows calculations from a standard transfer-matrix simulationperformed on an inverted PV device consistent with the embodimentsdescribed herein: quartz/Ni (1000 Å)/CuPc (400 Å)/PTCBI (100 Å)/BCP (100Å)/ITO (400 Å). The optical fields at peak absorption of CuPc at 625 nm(squares) and of PTCBI at 540 nm (stars) are shown.

FIG. 5a shows current-voltage curves for a control PV device grown onglass: ITO (1550 Å)/CuPc (200 Å)/PTCBI (250 Å)/BCP (100 Å)/Ag (1000 Å)in the dark (filled squares) and under simulated 1-sun illumination(open circles). FIG. 5a also shows current-voltage curves for aninverted PV device consistent with the embodiments described herein:quartz/Ni (1000 Å)/CuPc (400 ↑)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å) inthe dark (filled triangles) and under simulated 1-sun illumination (opentriangles). The lines are fit to the dark current curves.

FIG. 5b shows η_(P) (squares), V_(OC) (stars) and FF (triangles) as afunction of incident power density for an inverted PV device consistentwith the embodiments described herein: quartz/Ni (1000 Å)/CuPc (400Å)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å) treated with Ar plasma.

FIG. 6a shows the current-voltage characteristics for a control device(glass/ITO (1550 Å)/CuPc (200 Å)/PTCBI (250 Å)/BCP (100 Å)/Ag (1000 Å))in the dark (squares) and under simulated 1 sun, AM1.5G illumination(dashed lines), and for inverted PV device consistent with theembodiments described herein: quartz/Ni (1000 Å)/CuPc (350 Å)/PTCBI (100Å)/BCP (100 Å)/ITO (400 Å) in the dark (triangles) and underillumination (dashed-dotted line).

FIG. 6b shows the η_(P) (circles), V_(OC) (triangles), and FF (squares)for an inverted PV device comprising quartz/Ni (1000 Å)/CuPc (350Å)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å).

FIG. 7a shows the simulated (line) and measured (filled squares)photocurrent at one sun intensity of inverted PV devices having varyingCuPc thicknesses (x=100 to 400 Å) in a structure comprising quartz/Ni(1000 Å)/CuPc (x Å)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å).

FIG. 7b shows η_(P) (squares), V_(OC) (triangles), and FF at one sunAM1.5G illumination of inverted PV devices having varying CuPcthicknesses (x=100 to 400 Å) in a structure comprising quartz/Ni (1000Å)/CuPc (x Å)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å).

FIG. 8a shows the simulated (line) and measured (filled squares)photocurrent at one sun intensity of inverted PV devices having varyingPTCBI thicknesses (y=0 to 300 Å) in a structure comprising quartz/Ni(1000 Å)/CuPc (400 Å)/PTCBI (y Å)/BCP (100 Å)/ITO (400 Å).

FIG. 8b shows η_(P) (squares), V_(OC) (triangles), and FF at one sunAM1.5G illumination of inverted PV devices having varying PTCBIthicknesses (y=0 to 300 Å) in a structure comprising quartz/Ni (1000Å)/CuPc (400 Å)/PTCBI (y Å)/BCP (100 Å)/ITO (400 Å).

FIG. 9a shows a simulated contour plot for J_(SC) as a function of CuPcand PTCBI thicknesses.

FIG. 9b shows a simulated contour plot for η_(P) as a function of CuPcand PTCBI thicknesses.

DETAILED DESCRIPTION

Inverted organic photosensitive optoelectronic devices are describedherein. The organic devices described may be used, for example, togenerate a usable electrical current from incident electromagneticradiation (e.g., PV devices) or may be used to detect incidentelectromagnetic radiation. Some embodiments may comprise an anode, acathode, and a photoactive region between the anode and the cathode. Thephotoactive region is the portion of the photosensitive device thatabsorbs electromagnetic radiation to generate excitons that maydissociate in order to generate an electrical current. The devicesdescribed herein may also include at least one transparent electrode toallow incident radiation to be absorbed within the device. Several PVdevice materials and configurations are described in U.S. Pat. Nos.6,657,378, 6,580,027, and 6,352,777, which are incorporated herein byreference for their disclosure of PV device materials andconfigurations.

As used herein, the term “layer” refers to a member or component of aphotosensitive device whose primary dimension is X-Y, i.e., along itslength and width. It should be understood that the term layer is notnecessarily limited to single layers or sheets of materials. Inaddition, it should be understood that the surfaces of certain layers,including the interface(s) of such layers with other material(s) orlayers(s), may be imperfect, wherein said surfaces represent aninterpenetrating, entangled or convoluted network with other material(s)or layer(s). Similarly, it should also be understood that a layer may bediscontinuous, such that the continuity of said layer along the X-Ydimension may be disturbed or otherwise interrupted by other layer(s) ormaterial(s).

The terms “electrode” and “contact” are used herein to refer to a layerthat provides a medium for delivering photo-generated current to anexternal circuit or providing a bias current or voltage to the device.That is, an electrode, or contact, provides the interface between theactive regions of an organic photosensitive optoelectronic device and awire, lead, trace or other means for transporting the charge carriers toor from the external circuit. Anodes and cathodes are examples. U.S.Pat. No. 6,352,777, incorporated herein by for its disclosure ofelectrodes, provides examples of electrodes, or contacts, which may beused in a photosensitive optoelectronic device. In a photosensitiveoptoelectronic device, it may be desirable to allow the maximum amountof ambient electromagnetic radiation from the device exterior to beadmitted to the photoconductively active interior region. That is, theelectromagnetic radiation must reach a photoconductive layer(s), whereit can be converted to electricity by photoconductive absorption. Thisoften dictates that at least one of the electrical contacts should beminimally absorbing and minimally reflecting of the incidentelectromagnetic radiation. In some cases, such a contact should besubstantially transparent. The opposing electrode may be a reflectivematerial so that light which has passed through the cell without beingabsorbed is reflected back through the cell. As used herein, a layer ofmaterial or a sequence of several layers of different materials is saidto be “transparent” when the layer or layers permit at least about 50%of the ambient electromagnetic radiation in relevant wavelengths to betransmitted through the layer or layers. Similarly, layers which permitsome, but less than about 50% transmission of ambient electromagneticradiation in relevant wavelengths are said to be “semi-transparent.”

The term “cathode” is used in the following manner. In a non-stacked PVdevice or a single unit of a stacked PV device under ambient irradiationand connected with a resistive load and with no externally appliedvoltage, e.g., a PV device, electrons move to the cathode from thephoto-conducting material. Similarly, the term “anode” is used hereinsuch that in a PV device under illumination, holes move to the anodefrom the photoconducting material, which is equivalent to electronsmoving in the opposite manner. It will be noted that as the terms areused herein, anodes and cathodes may be electrodes or charge transferlayers.

As used herein, “top” means furthest away from the substrate structure(if present), while “bottom” means closest to the substrate structure.If the device does not include a substrate structure, then “top” meansfurthest away from the reflective electrode. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate structure, and is generally the first electrodefabricated. The bottom electrode has two surfaces, a bottom side closestto the substrate, and a top side further away from the substrate. Wherea first layer is described as “disposed over” or “on top of” a secondlayer, the first layer is disposed further away from substrate. Theremay be other layers between the first and second layer, unless it isspecified that the first layer is “in physical contact with” the secondlayer. For example, a cathode may be described as “disposed over” or “ontop of” an anode, even though there are various organic layers inbetween.

FIG. 1 shows inverted organic photosensitive optoelectronic device 100.The figures are not necessarily drawn to scale. Device 100 may includereflective substrate 110, donor layer 115, acceptor layer 120, optionalblocking layer 125, and transparent electrode 130. Device 100 may befabricated by depositing the layers described, in order. In someembodiments, the device described in FIG. 1 may optionally include avery thin, damage inducing metal layer between blocking layer 125 andtransparent electrode 130 such that transparency is not impacted. Device100 may also optionally include substrate structure 135. In someembodiments, the substrate structure may directly support reflectiveelectrode 110.

The specific arrangement of layers illustrated in FIG. 1 is exemplaryonly, and is not intended to be limiting. For example, some of thelayers (such as blocking layers) may be omitted. Other layers (such asreflective electrode or additional acceptor and donor layers) may beadded. The order of layers may be altered. Arrangements other than thosespecifically described may be used. Additionally, the organic PV devicemay exist as a tandem device comprising one or more additionaldonor-acceptor layers. A tandem device may have charge transfer layers,electrodes, or charge recombination layers between the tandemdonor-acceptor layers. The substrate and reflective electrode may becombined, the substrate may be reflective and the electrode transparent.

Substrate 135, onto which the device may be grown or placed, may be anysuitable material that provides the desired structural properties. Thesubstrate may be flexible or rigid, planar or non-planar. The substratemay be transparent, translucent or opaque. Plastic, glass, and quartzare examples of rigid substrate materials. Plastic and metal foils areexamples of flexible substrate materials. The material and thickness ofthe substrate may be chosen to obtain the desired structural and opticalproperties.

In some embodiments, reflective electrode 110 may comprise an electrode,such as a metal anode. In some embodiments, reflective electrode 110 maycomprise a low work function metal selected from steel, Ni, Ag, Al, Mg,In, and mixtures or alloys thereof. In some embodiments, the electrodemay comprise one metal as the base and one as the electrode material,such as Ti, stainless steel, or Al sheets, with or without Ag on top.

In some embodiments, reflective electrode 110 and substrate material 135may be combined or formed of two metals. In some embodiments substrate135 is reflective and electrode 110 is transparent.

In some embodiments, the “electrodes” described herein may be composedof “metal” or “metal substitutes.” Herein, the term “metal” is used toembrace both materials composed of an elementally pure metal, e.g., Mg,and also metal alloys which are materials composed of two or moreelementally pure metals, e.g., Mg and Ag together, denoted Mg:Ag. Here,the term “metal substitute” refers to a material that is not a metalwithin the normal definition, but which has the metal-like propertiesthat are desired in certain appropriate applications. Commonly usedmetal substitutes for electrodes and charge transfer layers wouldinclude doped wide-bandgap semiconductors, for example, transparentconducting oxides such as indium tin oxide (ITO), gallium indium tinoxide (GITO), and zinc indium tin oxide (ZITO). In particular, ITO is ahighly doped degenerate n+ semiconductor with an optical bandgap ofapproximately 3.2 eV, rendering it transparent to wavelengths greaterthan approximately 3900 Å. Another suitable metal substitute is thetransparent conductive polymer polyaniline (PANT) and its chemicalrelatives.

Metal substitutes may be further selected from a wide range ofnon-metallic materials, wherein the term “non-metallic” is meant toembrace a wide range of materials, provided that the material is free ofmetal in its chemically uncombined form. When a metal is present in itschemically uncombined form, either alone or in combination with one ormore other metals as an alloy, the metal may alternatively be referredto as being present in its metallic form or as being a “free metal”.Thus, the metal substitute electrodes described herein may sometimes bereferred to as “metal-free,” wherein the term “metal-free” is expresslymeant to embrace a material free of metal in its chemically uncombinedform. Free metals typically have a form of metallic bonding that resultsfrom a sea of valence electrons which are free to move in an electronicconduction band throughout the metal lattice. While metal substitutesmay contain metal constituents, they are “non-metallic” on severalbases. They are not pure free-metals nor are they alloys of free-metals.When metals are present in their metallic form, the electronicconduction band tends to provide, among other metallic properties, ahigh electrical conductivity as well as a high reflectivity for opticalradiation.

Transparent electrode 130 may be chosen from transparent oxides andmetal or metal substitutes having a thickness sufficient to render themtransparent. Commonly used metal substitutes for electrodes and chargetransfer layers would include doped wide-bandgap semiconductors, forexample, transparent conducting oxides. In some embodiments, transparentelectrode 130 may be selected from ITO, GITO, and ZITO. Other exemplaryelectrodes include highly transparent, non-metallic, low resistancecathodes such as those disclosed in U.S. Pat. No. 6,420,031, toParthasarathy et al., or a highly efficient, low resistancemetallic/non-metallic compound cathode such as those disclosed in U.S.Pat. No. 5,703,436 to Forrest et al., both incorporated herein byreference for their disclosure of cathodes. Each type of cathode istypically prepared in a fabrication process that includes the step ofsputter depositing an ITO layer onto either an organic material, such asCuPc, to form a highly transparent, non-metallic, low resistance cathodeor onto a thin Mg:Ag layer to form a highly efficient, low resistancemetallic/non-metallic compound cathode.

The devices described herein will comprise at least one “photoactiveregion” in which light is absorbed to form an excited state, or“exciton”, which may subsequently dissociate in to an electron and ahole. The dissociation of the exciton will typically occur at the“heterojunction” formed by the juxtaposition of an donor layer and anacceptor layer. For example, in the device of FIG. 1, the “photoactiveregion” may include donor layer 115 and acceptor layer 120. Chargeseparation may occur predominantly at the organic heterojunction betweendonor layer 115 and acceptor layer 120. The built-in potential at theheterojunction is determined by the HOMO-LUMO energy level differencebetween the two materials contacting to form the heterojunction. TheHOMO-LUMO gap offset between the donor and acceptor materials producesan electric field at the donor-acceptor interface that facilitatesdissociation of excitons created within an exciton diffusion length ofthe interface into opposite signed carriers (holes and electrons).

Suitable materials comprising acceptor layer 120 may include, forexample, polymeric or non-polymeric perylenes, naphthalenes, fullerenesor nanotubules. In some embodiments, acceptor layer 120 may comprise3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). In otherembodiments, acceptor layer 120 may comprise a fullerene material asdescribed in U.S. Pat. No. 6,580,027, the description of fullerenematerial which is incorporated herein by reference in its entirety. Insome embodiments, donor layer 115 may comprise squaraines,phthalocyanine, porphyrin, subphthalocyanine (SubPc), copperphthalocyanine (CuPc), or a derivative or transition metal complexthereof such as aluminum phthalocyanine chloride (AlClPc).

Other suitable organic materials for use in the photoactive layers mayinclude cyclometallated organometallic compounds. The term“organometallic” as used herein is as generally understood by one ofordinary skill in the art and as given, for example, in “InorganicChemistry” (2nd Edition) by Gary L. Miessler and Donald A. Tarr,Prentice Hall (1998). Thus, the term organometallic may refer tocompounds which have an organic group bonded to a metal through acarbon-metal bond. Organometallic compounds may comprise, in addition toone or more carbon-metal bonds to an organic species, one or more donorbonds from a heteroatom. The carbon-metal bond to an organic species mayrefer, for example, to a direct bond between a metal and a carbon atomof an organic group, such as phenyl, alkyl, alkenyl, etc. The termcyclometallated refers to compounds that comprise a bidentateorganometallic ligand so that, upon bonding to a metal, a ring structureis formed that includes the metal as one of the ring members.

As alluded to above with respect to the term “layer,” it should beunderstood that the boundary of acceptor layer 120 and donor layer 115,as depicted in FIG. 1, may be imperfect, discontinuous, and/or otherwiserepresent an interpenetrating, entangled or convoluted network of donorand acceptor materials. For example, in some embodiments, while theorganic donor-acceptor heterojunction may form a planar heterojunction,in others it may form a bulk heterojunction, nanocrystalline bulkheterojunction, hybrid planar-mixed heterojunction, or mixedheterojunction. In some embodiments, two or more organic donor-acceptorheterojunctions may be used to create a tandem inverted PV device.

Organic layers may be fabricated using vacuum deposition, spin coating,organic vapor-phase deposition, inkjet printing, and other methods knownin the art.

Organic photosensitive optoelectronic devices of the embodimentsdescribed herein may function as a PV device, photodetector orphotoconductor. Whenever the organic photosensitive optoelectronicdevices described herein function as a PV device, the materials used inthe photoconductive organic layers and the thicknesses thereof may beselected, for example, to optimize the external quantum efficiency ofthe device. Whenever the organic photosensitive optoelectronic devicesdescribed herein function as photodetectors or photoconductors, thematerials used in the photoconductive organic layers and the thicknessesthereof may be selected, for example, to maximize the sensitivity of thedevice to desired spectral regions.

The desired result may be achieved by considering several guidelinesthat may be used in the selection of layer thicknesses. It may bedesirable for the layer thickness, L, to be less than or on the order ofan exciton diffusion length, L_(D), since it is believed that mostexciton dissociation will occur within a diffusion length of adonor-acceptor interface. In this description, L is the distance fromthe exciton formation site and a donor-acceptor interface. If L isgreater than L_(D), then many excitons may recombine beforedissociation. It is further desirable for the total photoconductivelayer thickness to be of the order of the electromagnetic radiationabsorption length, 1α (where α is the absorption coefficient), so thatnearly all of the radiation incident on the PV device is absorbed toproduce excitons. Furthermore, the photoconductive layer thicknessshould be as thin as possible to avoid excess series resistance due tothe high bulk resistivity of organic semiconductors.

Accordingly, such competing guidelines inherently may require tradeoffsto be made in selecting the thickness of the photoconductive organiclayers of a photosensitive optoelectronic cell. Thus, on the one hand, athickness that is comparable or larger than the absorption length may bedesirable in order to absorb the maximum amount of incident radiation.On the other hand, as the photoconductive layer thickness increases, twoundesirable effects may be increased. One may be due to the high seriesresistance of organic semiconductors, as an increased organic layerthickness may increase device resistance and reduce efficiency. Anotherundesirable effect is that increasing the photoconductive layerthickness may increase the likelihood that excitons will be generatedfar from the charge-separating interface, resulting in enhancedprobability of geminate recombination and, again, reduced efficiency.Therefore, it may be desirable to have a device configuration thatbalances between such competing effects, in a manner that produces ahigh external quantum efficiency for the overall device.

The device of FIG. 1 may further include one or more of blocking layer125, such as the exciton blocking layers (EBLs) described in U.S. Pat.No. 6,097,147, Peumans et al., Applied Physics Letters 2000, 76,2650-52, and U.S. Pat. No. 6,451,415, Forrest et al., all of which areincorporated herein by reference for their disclosure of blockinglayers. In certain embodiments, higher internal and external quantumefficiencies have been achieved by the inclusion of an EBL to confinephotogenerated excitons to the region near the dissociating interfaceand to prevent parasitic exciton quenching at a photosensitiveorganic/electrode interface. In addition to limiting the volume overwhich excitons may diffuse, an EBL can also act as a diffusion barrierto substances introduced during deposition of the electrodes. In somecircumstances, an EBL can be made thick enough to fill pinholes orshorting defects which could otherwise render an organic PV devicenon-functional. An EBL can therefore help protect fragile organic layersfrom damage produced when electrodes are deposited onto the organicmaterials.

Without being bound to any particular theory, it is believed that theEBLs derive their exciton blocking property from having a LUMO-HOMOenergy gap substantially larger than that of the adjacent organicsemiconductor from which excitons are being blocked. Thus, the confinedexcitons are prohibited from existing in the EBL due to energyconsiderations. While it is desirable for the EBL to block excitons, itis not desirable for the EBL to block all charge. However, due to thenature of the adjacent energy levels, an EBL may block one sign ofcharge carrier. By design, an EBL will exist between two other layers,usually an organic photosensitive semiconductor layer and an electrode,a charge transfer layer or a charge recombination layer. The adjacentelectrode or charge transfer layer will be in context either a cathodeor an anode. Therefore, the material for an EBL in a given position in adevice will be chosen so that the desired sign of carrier will not beimpeded in its transport to the electrode or charge transfer layer.Proper energy level alignment ensures that no barrier to chargetransport exists, preventing an increase in series resistance. Incertain embodiments, it may be desirable for a material used as acathode side EBL to have a LUMO energy level closely matching the LUMOenergy level of the adjacent acceptor material so that any undesiredbarrier to electrons is minimized.

It should be appreciated that the exciton blocking nature of a materialis not necessarily an intrinsic property of its HOMO-LUMO energy gap.Whether a given material will act as an exciton blocker depends upon therelative HOMO and LUMO energy levels of the adjacent organicphotosensitive material. Therefore, it may not be possible to identify aclass of compounds in isolation as exciton blockers without regard tothe device context in which they may be used. However, with theteachings herein, one of ordinary skill in the art may identify whethera given material will function as an exciton blocking layer when usedwith a selected set of materials to construct an organic PV device.

In some embodiments, blocking layer 125 may comprise an EBL situatedbetween acceptor layer 120 and transparent electrode 130. Examples ofsuitable EBL materials include, but are not limited to,2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproinor BCP), which is believed to have a LUMO-HOMO energy level separationof about 3.5 eV, orbis(2-methyl-8-hydroxyquinolinoato)-aluminum(III)phenolate (Alq₂OPH).BCP may be an effective exciton blocker which can easily transportelectrons to the cathode from an acceptor layer. In other embodiments,the EBL may be selected fromN,N′-diphenyl-N,N′-bis-alpha-naphthylbenzidine (NPD), aluminum tris(8-hydroxyquinoline) (Alq3), carbazole biphenyl (CBP), andtris(acetylacetonato) ruthenium (III) (Ru(acac)₃).

In some embodiments, blocking layer 125 may comprise an EBL doped with asuitable dopant, including but not limited to3,4,9,10-perylenetracarboxylic dianhydride (PTCDA),3,4,9,10-perylenetracarboxylic diimide (PTCDI),3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI),1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), and derivativesthereof. BCP, as deposited in the devices described herein, may beamorphous. Amorphous BCP exciton blocking layers may exhibit filmrecrystallization, which may be especially rapid under high lightintensities. The resulting morphology change to polycrystalline materialresults in a lower quality film with possible defects such as shorts,voids or intrusion of electrode material. Accordingly, it has been foundthat doping of some EBL materials, such as BCP, that exhibit this effectwith a suitable, relatively large and stable molecule can stabilize theEBL structure to prevent performance degrading morphology changes. Itshould be further appreciated that doping of an EBL which istransporting electrons in a given device with a material having a LUMOenergy level close to that of the EBL may help to insure that electrontraps are not formed which might produce space charge build-up andreduce performance. Additionally, it should be appreciated thatrelatively low doping densities should minimize exciton generation atisolated dopant sites. Since such excitons are effectively prohibitedfrom diffusing by the surrounding EBL material, such absorptions reducedevice photoconversion efficiency.

In some embodiments, the device of FIG. 1 may further comprise one ormore transparent charge transfer layers or charge recombination layers.As described herein, charge transfer layers are distinguished fromacceptor and donor layers by the fact that charge transfer layers arefrequently, but not necessarily, inorganic (often metals) and they maybe chosen not to be photoconductively active. The term “charge transferlayer” is used herein to refer to layers similar to but different fromelectrodes in that a charge transfer layer only delivers charge carriersfrom one subsection of an optoelectronic device to the adjacentsubsection. The term “charge recombination layer” is used herein torefer to layers similar to but different from electrodes in that acharge recombination layer allows for the recombination of electrons andholes between tandem photosensitive devices and may also enhanceinternal optical field strength near one or more active layers. A chargerecombination layer can be constructed of semi-transparent metalnanoclusters, nanoparticles or nanorods as described in U.S. Pat. No.6,657,378, the disclosure of which is incorporated herein by reference.

In other embodiments, a smoothing layer may be situated betweenreflective electrode 110 (e.g., anode) and donor layer 115. A exemplarymaterial for this layer comprises a film of3,4-polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS). Theintroduction of the PEDOT:PSS layer between reflective electrode 110(e.g., anode comprising ITO) and donor layer 115 (e.g., CuPc) may leadto greatly improved fabrication yields. Without being bound to aparticular theory, it is believed that the improved fabrication yieldsmay be a result of the ability of the spin-coated PEDOT:PSS film toplanarize the ITO, whose rough surface could otherwise result in shortsthrough the thin molecular layers.

In a further embodiment, one or more of the layers of the FIG. 1 devicemay undergo surface treatments. For example, one or more of the layersmay be treated with plasma prior to depositing the next layer. Thelayers may be treated, for example, with a mild argon or oxygen plasma.This treatment may be beneficial in reducing the series resistance. Itmay be advantageous to subject an optional PEDOT:PSS layer to a mildplasma treatment prior to deposition of the next layer. Alternatively,one or more of the layers may be exposed to ultra-violet ozone (UV-O₃)treatment. In at least one embodiment, the reflective electrode (e.g.,anode layer) is exposed to a surface treatment.

The embodiments described herein also include a method for producing theorganic PV device of FIG. 1, comprising: providing reflective electrode110, performing at least one surface treatment on reflective electrode110, forming an organic donor-acceptor heterojunction (e.g., donor layer115 and acceptor layer 120) over reflective electrode 110, and formingtransparent electrode 130 over said organic donor-acceptorheterojunction.

The embodiments described herein also include methods for generatingand/or measuring electricity. In some embodiments, that methodcomprises: providing light to the device of FIG. 1, which comprisesreflective electrode 110, organic donor-acceptor heterojunction on topof said reflective electrode (e.g., donor layer 115 and acceptor layer120), and transparent electrode 130 on top of said donor-acceptorheterojunction.

In some embodiments, a power-generating device is described, which mayinclude at least one device of FIG. 1, comprising: a reflectiveelectrode 110; organic donor-acceptor heterojunction on top of saidreflective electrode (e.g., donor layer 115 and acceptor layer 120); andtransparent electrode 130 on top of said donor-acceptor heterojunction.In some embodiments, the device may be in the form of a paint, film, orfoil. For example, in one embodiment, device 100 can be formed onsubstrate structure 135, which comprises a film, foil, or the like, orformed directly on the enclosure of a device, such as applying paint. Insome embodiments, the device displays a qp in a range from about 0.3 toabout 0.4. In some embodiments, the device displays a V_(oc) in a rangefrom about 0.2 to about 1.5, such as about 0.4 to about 0.5. In someembodiments, the device displays a FF in the range of about 0.4 to about0.85, such as about 0.5. In some embodiments, the device displaysJ_(sc)/P₀ in the range of about 0.002 to about 0.025 Å/W, such as 0.02.In some embodiments, the device displays a R_(SA) in a range from about5 to about 12. In some embodiments, the device displays a J_(S) in arange from about 2×10⁻⁷ to about 7×10⁻⁷. In some embodiments, the devicedisplays an η of less than about 2, such as approaching about 1.

In further embodiments, the organic photosensitive optoelectronicdevices described herein may function as photodetectors. In thisembodiment, device 100 may be a multilayer organic device, for example,as described in U.S. Pat. No. 6,972,431, the disclosure of which isincorporated herein by reference. In this case, an external electricfield may be generally applied to facilitate extraction of the separatedcharges.

Coatings may be used to focus optical energy into desired regions ofdevice 100. See, e.g., U.S. Pat. No. 7,196,835; U.S. patent applicationSer. No. 10/915,410, which is incorporated by reference to provideexamples of such a coating.

The simple layered structure illustrated in FIG. 1 is provided by way ofnon-limiting example, and it is understood that embodiments describedherein may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional organic photosensitive optoelectronic devices may be achievedby combining the various layers described in different ways, or layersmay be omitted entirely, based on design, performance, and cost factors.Other layers not specifically described may also be included. Materialsother than those specifically described may be used. Although many ofthe examples provided herein describe various layers as comprising asingle material, it is understood that combinations of materials, suchas a mixture of host and dopant, or more generally a mixture, may beused. Also, the layers may have various sublayers. The names given tothe various layers herein are not intended to be strictly limiting.Organic layers that are not a part of the photoactive region, i.e.,organic layers that generally do not absorb photons that make asignificant contribution to photocurrent, may be referred to as“non-photoactive layers.” Examples of non-photoactive layers includeEBLs and anode-smoothing layers. Other types of non-photoactive layersmay also be used.

The devices described herein will be further described by the followingnon-limiting examples, which are intended to be purely exemplary.

EXAMPLES Example 1

Inverted structures were demonstrated using the archetype donor-acceptorbilayer system formed by CuPc and PTCBI. Optical simulations wereemployed to predict device performance and optimize the invertedstructures. Standard transfer-matrix calculations were performed topredict the J_(SC). See, e.g., Appl. Phys. Rev. 93, 3693 (2003) and J.Appl. Phys. 86, 487 (1999), which are incorporated herein by referencefor the disclosure of transfer-matrix calculations. Results of thethickness studies are shown in FIGS. 2 and 3.

Example 2

Optical constants of organic films grown on Si substrates were measuredusing ellipsometry, while those of Ni were taken from the literature.See, e.g., J. Phys. F: Metal Phys. 9, 2491 (1979), which is incorporatedherein by reference for this purpose. Exciton diffusion lengths of CuPcand PTCBI were taken to be 80 Å and 40 Å, respectively, with lifetimesof 2 ns. See, e.g., Appl. Phys. Rev. 93, 3693 (2003), which isincorporated herein by reference for this purpose. In the simulations,three structures were investigated: one control PV and two inverted PVs.The control PV device was glass/ITO (1550 Å)/CuPc (200 Å)/PTCBI (250Å)/BCP (100 Å)/Ag (1000 Å). Results for this control PV can be seen inFIG. 4a . The first inverted PV device was quartz/Ag (1000 Å)/BCP (100Å)/PTCBI (300 Å)/CuPc (150 Å)/ITO (400 Å). Results for this control PVcan be seen in FIG. 4b . The second inverted PV device was quartz/Ni(1000 Å)/CuPc (400 Å)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å). Results forthis control PV can be seen in FIG. 4 c.

Example 3

Three different types of the second inverted PV device (quartz/Ni (1000Å)/CuPc (400 Å)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å)) were formed bypreparing a quartz substrate by solvent cleaning. See, e.g., OrganicElectron 6, 242 (2005), which is incorporated herein by references forthis purpose. The quartz base structures were loaded into an electronbeam evaporator where 1000 Å Ni were deposited at a rate of 10 Å/s. See,e.g., Appl. Phys. Lett. 86, 263502 (2005), which is incorporated hereinby references for this purpose. The Ni anodes were exposed to threedifferent surface treatments. The first was exposed to 30 minutes ofultra-violet ozone (UV-O₃) treatment. The second was exposed to O₂plasma at a power of 120 W for 80 s. The third was exposed to Ar plasmaat a power of 70 W for 2 minutes, followed by 30 minutes of UV-O₃treatment. The structures were then loaded in a high vacuum thermaldeposition chamber having base pressure of 5×10⁻⁷ Torr. Purified organicsources were grown at a pressure of 1×10⁻⁶ Torr and a rate of 2 Å/s.See, e.g., Organic Electron 6, 242 (2005), which is incorporated hereinby reference for this purpose. The planar double heterojunction solarcell PV structure grown consisted of a 400 Å thick CuPc donor layer, a100 Å thick PTCBI acceptor layer, and a 1000 Å thick BCP excitonblocking (see, e.g., Appl. Phys. Lett. 76, 2650 (2000), which isincorporated herein by reference for this purpose) and damage absorbinglayer. A vacuum break and exposure to air was necessary before attachinga shadow mask in a nitrogen ambient atmosphere. The top contact wasformed by a 400 Å thick ITO cathode layer sputter-deposited at 15 W and13.56 MHz through the shadow mask defining 1 mm diameter holes.

Current-voltage measurements were used to characterize the performanceof the cells in the dark and under simulated AM1.5G solar illumination(uncorrected for solar spectral mismatch) using a 150 W Xenon arc lamp.Performance data for the Ar plasma treated device is shown in FIG. 5.The dark current (filled triangles) and 1 sun illumination (opentriangles) current-voltage curves are shown in FIG. 5a , along with thefit to the dark current (line). Performance for this cell as a functionof illumination intensity is shown in FIG. 5 b.

The control PV was grown under similar conditions, in the same chamberand using the same organic materials, on solvent-cleaned, 10 minuteUV-O₃ treated ITO-coated glass, comprising: ITO (1550 Å)/CuPc (200Å)/PTCBI (250 Å)/BCP (100 Å)/Ag (1000 Å). Under AM1.5G 1 sun solarillumination, the control device displayed a V_(OC) of 0.44 V, a FF of0.64, a J_(SC)/P₀ of 0.44 Å/W, leading to a η_(P) of 1.2±0.1%. The darkcurrent-voltage current curve was fit to the modified ideal diodeequation:

$J_{D} = {J_{S}\left\{ {{\exp\left\lbrack \frac{q\left( {V - {J_{D}R_{SA}}} \right)}{nkT} \right\rbrack} - 1} \right\}}$

giving n of 1.66, R_(SA) of 0.75 Ω-cm², and J_(S) of 9.8×10⁻⁸ Å/cm².FIG. 5a shows the device current in the dark (filled squares) and under1 sun illumination (open circles). The line represents the fit to thedark current.

Table 1 (below) lists dark curve fit parameters and AM1.5G 1 sunperformance data for devices grown on substrates exposed to the threedifferent surface treatments discussed above.

TABLE 1 Surface 1 sun η_(P) 1 sun 1 sun J_(S)/P_(O) R_(SA) Treatment (%)V_(OC) (V) 1 sun FF (A/W) (Ω-cm²) J_(S) (A/cm²) n UV-O₃ 0.31 ± 0.39 ±0.50 ± 0.016 ± 9.1 ± 2.1 × 10⁻⁷ ± 1.88 ± 0.06 0.03 0.05 0.001 4.6 0.9 ×10⁻⁷ 0.08 O₂ plasma 0.31 ± 0.37 ± 0.51 ± 0.016 ± 4.9 ± 7.0 × 10⁻⁷ ± 2.00± 0.02 0.01 0.01 0.002 1.0 0.5 × 10⁻⁷ 0.10 Ar plasma 0.35 ± 0.45 ± 0.48± 0.017 ± 11.6 ± 2.7 × 10⁻⁸ ± 1.83 ± 0.04 0.01 0.03 0.001 3.7 0.2 × 10⁻⁸0.10

Example 4

Inverted PV devices having CuPc and PTCBI layers of varying thicknesswere prepared as follows. Quartz substrates were solvent cleaned, thenloaded into an electron beam evaporator where a 1000 Å thick layer of Niwas deposited at a rate of 5 Å/s. The Ni anodes were exposed toultraviolet ozone treatment for 30 min, then loaded into a high vacuumthermal deposition chamber with a base pressure of 5×10 Torr. Purifiedorganic sources were grown at a pressure of 1×10⁻⁶ Torr, and a rate of 2Å/s. A vacuum break occurred before attaching a shadow mask to thedeposited layers and substrate in a high purity (<1 ppm H₂O and O₂) N₂ambient. The top contact consisted of a 400 Å thick ITO layer sputterdeposited at 20 W with a rate of 0.1 Å/s through the shadow maskdefining an array of 1 mm diameter circular cathodes. Performance datafor the devices prepared by this method are disclosed below.

The performance of an inverted PV device comprising quartz/Ni (1000Å)/CuPc (350 Å)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å) is disclosed inFIG. 6. FIG. 6a shows the current-voltage characteristics for a controldevice (glass/ITO (1550 Å)/CuPc (200 Å)/PTCBI (250 Å)/BCP (100 Å)/Ag(1000 Å)) in the dark (squares) and under simulated 1 sun, AM1.5Gillumination (dashed lines), and for the inverted device in the dark(triangles) and under illumination (dashed-dotted line). FIG. 6b showsthe power conversion efficiency (circles), open-circuit voltage(triangles), and fill factor (squares) for the inverted device.

FIGS. 7 and 8 show the performance of inverted devices having CuPclayers (x=100 to 400 Å) and PTCBI layers (y=0 to 300 Å) of varyingthickness. The structures of FIG. 7 comprise quartz/Ni (1000 Å)/CuPc (xA)/PTCBI (100 Å)/BCP (100 Å)/ITO (400 Å). FIG. 7a shows the simulated(line) and measured (filled squares) photocurrent of such devices at onesun intensity. FIG. 7b shows η_(P) (squares), Voc (triangles), and FF atone sun AM1.5G illumination. The structures of FIG. 8 comprisequartz/Ni(1000 Å)/CuPc (400 Å)/PTCBI (y A)/BCP (100 Å)/ITO (400 Å). FIG.8a shows the simulated (line) and measured (filled squares) photocurrentof such devices at one sun intensity. FIG. 8b shows η_(P) (squares), Voc(triangles), and FF at one sun AM1.5G illumination.

FIG. 9 shows a simulated contour plot for a devices having varying CuPcand PTCBI thicknesses. FIG. 9a shows a simulated contour plot forJ_(SC), while FIG. 9b shows a simulated contour plot for η_(P). For FIG.9, it is assumed the ideality factor n=1.78, reverse saturation currentJ_(S)=1.46×10⁻⁷ Å/cm², and the series resistance as a function of theCuPc and PTCBI thicknesses (t_(CuPc) and t_(PTCBI)) followsR_(SA)=0.05×[(t_(CuPc)+t_(PTCBI))/Å]Ωcm². The diffusion lengths of CuPcand PTCBI were taken as 80±20 and 30±5 Å, with a lifetime of 2 ns.Values of Ni anode, ITO cathode. and BCP layers thicknesses were 1000,400, and 100 Å, respectively, for these simulations.

Although the present disclosure is described with respect to particularexamples and embodiments, it is understood that the devices describedherein are not limited to these examples and embodiments. Theembodiments as claimed may therefore include variations from theparticular examples and preferred embodiments described herein, as willbe apparent to one of skill in the art.

1-16. (canceled)
 17. A method for producing an inverted photosensitivedevice, said method comprising: providing a reflective electrode;performing at least one surface treatment on said reflective electrode;forming an organic donor-acceptor heterojunction over said reflectiveelectrode; and forming a transparent electrode over said organicdonor-acceptor heterojunction.
 18. The method of claim 17, wherein thereflective electrode is positioned over a substrate.
 19. The method ofclaim 17, wherein the donor of the organic donor-acceptor heterojunctioncomprises a material selected from phthalocyanines, porphyrins,subphthalocyanines, and derivatives or transition metal complexesthereof.
 20. The method of claim 17, wherein the donor of thedonor-acceptor heterojunction comprises copper phthalocyanine.
 21. Themethod of claim 17, wherein the acceptor of the organic donor-acceptorheterojunction comprises a material selected from polymeric ornon-polymeric perylenes, naphthalenes, and fullerenes.
 22. The method ofclaim 17, wherein the acceptor of the organic donor-acceptorheterojunction comprises 3,4,9,10-perylenetetracarboxylicbis-benzimidazole.
 23. The method of claim 17, wherein the transparentelectrode comprises a material selected from transparent oxides andmetal or metal substitutes.
 24. The photosensitive device of claim 17,wherein the transparent electrode permits at least about 50% of ambientelectromagnetic radiation to be transmitted through said electrode. 25.The method of claim 17, wherein the transparent electrode comprises amaterial selected from tin oxide, gallium indium tin oxide, and zincindium tin oxide.
 26. The method of claim 17, further comprising thestep of providing an exciton blocking layer.
 27. The method of claim 26,wherein the exciton blocking layer is positioned between the reflectivesubstrate and the transparent electrode.
 28. The method of claim 26,wherein the exciton blocking layer is positioned between the acceptor ofthe organic donor-acceptor heterojunction and the transparent electrode.29. The method of claim 26, wherein the exciton blocking layer comprisesa material selected from N,N′-diphenyl-N,N′-bis-alpha-naphthylbenzidine,aluminum tris (8-hydroxyquinoline), carbazole biphenyl, bathocuproine,and tris(acetylacetonato) ruthenium (III).
 30. The method of claim 17,wherein the organic donor-acceptor heterojunction comprises a structureselected from planar heterojunctions, bulk heterojunctions,nanocrystalline bulk heterojunctions, hybrid planar-mixedheterojunctions, and mixed heterojunctions.
 31. The method of claim 17,wherein the at least one surface treatment is selected from ultra-violetozone treatment, oxygen plasma treatment, and argon plasma treatment.